Feedstock materials for semi-solid forming

ABSTRACT

Disclosed is a method for producing feedstock materials for semi-solid forming comprising reducing the size of raw material, introducing the raw material into an extruder and extruding the raw material to produce feedstock material having a non-dendritic microstructure. Further disclosed is an apparatus for producing feedstock materials for semi-solid forming comprising an extruder, a feeder with means for maintaining raw material under a protective atmosphere, a nozzle on an end of the extruder with means for maintaining an extrudate under a protective atmosphere, temperature control means within the extruder and a pelletizer for pelletizing the extrudate. Additionally disclosed is a system for producing feedstock materials for semi-solid forming comprising a size reduction station, a magnetic screen, an extruder comprising a semi-solid raw material and a pelletizer.

BACKGROUND Field of the Invention

The present invention relates to feedstock materials for semi-solidforming, preferably feedstock materials comprising processed,reprocessed or materials to be recycled from semi-solid formingprocesses.

Metal alloys for semi-solid forming are known in the art. Typically,when magnesium alloys are used for thixomolding, the feedstock isproduced by chipping bars comprising magnesium. The chips from thesebars are, however, problematic because they are often contaminated withcutting fluid from the chipper and because they often contain metalcontaminates from the knives of a chipper. These chips cause additionalproblems when used as feedstock because the size and shapes of the chipsare non-uniform. As a result of the non-uniform size and shape of thechips, (1) voids form in injection molded parts because of non-uniformpacking of the chips, (2) bridging may occur at the feed port, and/or(3) non-optimal performance of the injection molding apparatus occurs.Bridging at the feed port occurs because chips are of non-uniformthickness and therefore have non-uniform melting profiles. That is tosay, the thinner edges of the chips will melt before the thickercenters. From time to time, plural chips fuse together and become lodgedin entry and exit points, thereby blocking the flow of material andinterrupting the process. With regard to non-optimal performance of theapparatus, “hiccups” may occur when chips become lodged between anextruder screw and the wall of the extruder. This lodging often createsa whining sound and causes excessive wear to the screw and the barrelwall of the semi-solid forming machine.

Another problem lies with the microstructure of the feedstock materials.Traditional feedstock materials commonly have a dendritic microstructurethat is disadvantageous during the thixomolding operation. This problemis common in virgin feedstocks and may be encountered in recycledfeedstocks as well.

Another problem is with scraps produced during the thixoformingoperation. In a typical operation, scraps, such as sprues and runnersare returned to metal foundries, where the scraps are melted to a liquidstage, purified, realloyed, made into billets again (typically with amicrostructure containing dendrites not preferred for thixomolding),rechipped and sold back to thixomolder end users. Besides thedisadvantages of thixomolding feedstock materials having non-desirablemicrostructure and the necessity to have to transport scraps back tofoundries for reprocessing, the current techniques of recycling providea disincentive against using proprietary metallurgical compositions.

As a result, there exists a need for a feedstock material that avoidsthe problems discussed above as well as for an apparatus and process formaking such a feedstock.

SUMMARY

One object of the present invention is to provide a feedstock thatovercomes the problems and disadvantages of present feedstock materials,as well as a process and apparatus for feedstock production. Anadditional object of the invention is to overcome obstacles and costassociated with the recycling of thixotropic revert due to irregularshape and potential for contamination. Another object of the inventionis to produce a feedstock of a uniform shape which will feed well insubsequent processing. Yet another object of the invention is to providea process and apparatus for removing volatile paints and oils from therecycled material used as raw material to provide the feedstock in aflashing step. Yet another object of the invention is to provide aprocess and apparatus for further alloying elements into spent alloys inthe recycling and/or processing stage. Still another object of theinvention is to provide an inexpensive means to reduce raw material,including ingots, to a uniform raw material for processing to feedstockwithout going through the traditional chipping process and with onlyminimal contamination from a size reduction process. A further object ofthe invention is to produce a feedstock having a microstructure that isfavorable to thixoforming, which may include a globular structure or acrystalline microstructure.

In order to achieve the foregoing and further objects, there has beenprovided according to one aspect of the invention a process forproducing feedstock materials for semi-solid forming. The processdesirably includes providing solid-state raw material, reducing the sizeof the raw material to produce particles of raw material of a sizesuitable for further processing, introducing the particles of rawmaterial into an extruder and extruding particles of raw material toform an extrudate feedstock material. It may be desirable to separatethe particles of raw material by size, weight, density or other physicalcharacteristics before introducing the particles into the extruder. Theextrudate feedstock desirably includes a non-dendritic microstructuresuch as a globular microstructure.

According to another aspect of the invention, there has been provided anapparatus for producing feedstock materials for semi-solid metalforming. The apparatus generally includes an extruder, a feeder attachedto and in communication with the extruder for introducing raw materialinto the extruder, and means for maintaining the raw material under aninert atmosphere or vacuum in the feeder and as the raw material is fedinto the extruder. The apparatus may further include a discharge orifice(e.g., die or nozzle) affixed to the extruder and through which the rawmaterial exits the extruder as extrudate. The apparatus may also includea means for controlling the temperature of the extrudate while in theextruder and while the extrudate is being discharged from the extruder.Preferably, the extrudate reaches a temperature sufficient to transformthe raw material to its semi-solid state below its liquefyingtemperature. The apparatus desirably includes a means for maintainingthe extrudate under an inert atmosphere as the raw materials are beingextruded into the extrudate, as the extrudate exits the dischargeorifice and a pelletizer for dividing extrudate into pellets.

According to yet another aspect of the invention there has been provideda system for producing feedstock materials for semi-solid forming. Thesystem may include raw material, a size reduction station for reducingthe raw materials into particles, a magnetic screen for removingparticles larger in size than openings in the screen and for removingmagnetic contaminants, an extruder comprising a screw and containingsemi-solid raw material and a pelletizer.

Further objects, features and advantages of the present invention willbecome apparent from the detailed description of preferred embodimentsthat follows when considered together with the accompanying drawings. Itis to be understood that both the foregoing summary of the invention andthe following description of the drawings and detailed description areof a preferred embodiment, and not restrictive of the invention or otheralternate embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained below with reference to the exemplaryembodiments and with reference to the accompanying drawings, in which:

FIG. 1 depicts the process flow of raw material in accordance with anexemplary embodiment of the invention.

FIG. 2 depicts an extruder screw design in accordance with an exemplaryembodiment of the invention.

FIG. 3 is a micrograph depicting a dendritic microstructure.

FIG. 4 is a micrograph depicting a globular microstructure.

FIG. 5 depicts a pelletizer in the form of a rotary knife apparatus inaccordance with an exemplary embodiment of the invention.

FIG. 6 depicts a pelletizer in the form of a rotary cutter apparatus inaccordance with an exemplary embodiment of the invention.

FIG. 7 depicts a pelletizer in the form of a non-mechanical pelletizerin accordance with an exemplary embodiment of the invention.

FIG. 8 is a micrograph of a cross-section of feedstock material ofmagnesium alloy produced in accordance with the present invention at 50×magnification.

FIG. 9 is a micrograph of a cross-section of feedstock material ofmagnesium alloy produced in accordance with the present invention at100× magnification.

FIG. 10 is a micrograph of a cross-section of feedstock material of zincalloy produced in accordance with the present invention at 140×magnification.

DETAILED DESCRIPTION

The present invention provides a process to produce feedstock materialsfor semi-solid forming which have a preferred metallurgy,microstructure, and particle shape. Feedstock materials may comprisemagnesium alloys such as AZ91D, AM50, or AM60, and may also includecurrently non-thixomoldable magnesium alloys as well as currentlynon-thixomoldable non-magnesium alloys including, without limitation,zinc, aluminum, tin-lead and gallium based alloys. Raw materials forproducing the feedstock material commonly are in a solid-state, and mayinclude ingots, scrap from die casters or scrap, including scrap fromthixomolders such as sprues or runners. The raw material may becontaminated with organic paints, coatings, oils, and other non-metalcontaminates that typically volatilize at temperatures around 500° C.The raw material may comprise metal having a dendritic and/or anon-dendritic structure. In a preferred embodiment, at least some of theraw material may be recycled from previous thixoforming operations.

According to one aspect of the invention, a process for producing afeedstock material from raw materials is provided. A preferred processis shown in FIG. 1 and includes subjecting the raw material to sizereduction, mechanical separation and/or magnetic separation,microstructural and metallurgical refinement in a semi-solid state byextrusion and preferably pellet formation.

According to the preferred embodiment, raw materials in the form ofscrap and/or virgin magnesium may be reduced in size by shredding rawmaterials in, for example, a shear shredding machine to provideparticles of raw material. The particles may be of varying size and aregenerally no larger than the size of the feedthroat of the extruder,which is commonly no more than about 3″ in size, and more commonly nogreater than about 1″ in size, and more desirably no greater than about⅜″ in size. The particles may also comprise more fine particles of about¼″ or less in size. Generally, the size of the particles is dependant onthe equipment used to further process the raw material to feedstock aswould apparent to one skilled in the art. A shearing machine may beprovided which is capable of reducing raw materials such as scrap,revert and ingots to a uniform, particle size that may be fed into anextruder. In an embodiment that desires to achieve particle size ofabout ⅜″ or less, a shear shredding machine may, for example, haveintermeshing rotary knives spaced apart by about ⅜″ such that incomingraw material can be reduced to particles of about ⅜″ or less in size.According to another embodiment of the present invention, the rawmaterial may be reduced by the size reduction apparatus to particles ofabout ¼″ in size or less.

Other embodiments may include a different style of size reductionapparatus. For example, two shearing sets may be arranged in series. Inthis exemplary embodiment, a first set of blades (e.g., rotary knives)may reduce incoming raw material to particles of a first size (e.g.,about ½″ or less in size), and a second set of blades may reduce theparticles produced by the first set of blades to particles of a secondsize (e.g., about ⅜″ or less in size). According to yet anotherembodiment, raw materials may be reduced by a size reduction apparatusthat includes two plates (preferably contoured plates) that can be drawntogether to crush the raw materials to a desired size. The crushingapparatus may also include a heating system and an inert or protectiveatmosphere.

Once incoming raw material has been reduced by the shredder or othersize reducing apparatus, the particles may be separated based uponphysical characteristics of the particles. Common characteristicsinclude size, weight, shape, density, magnetism, or other physicalcharacteristic. Separation may be performed by one or more steps, asdesired. For example, the particles may be reduced and separated by apredetermined size, such as about ⅜″ or less. Separation may be employedwith magnetized screens to remove magnetic contaminants, such as ferrouscontaminants and prevent passage of particles greater than about ⅜″. Themagnetic contaminants may have entered during size reduction or havebeen a part of the raw materials. If magnetizing screens are used toremove magnetic contaminants, the screens are preferably interchangeablebecause they can be cleaned frequently and provide for maximumefficiency in removing magnetic contaminants.

In the preferred embodiment as shown in FIG. 1, non-magnetic screens maybe used in addition to magnetic screens. Regardless of whether anon-magnetic and/or a magnetic screen is employed, the screen alsoensures that material of a predetermined size, such as about ⅜″, orlarger does not move into the next stage of the process. The size ofmaterial excluded from further processing by the screen depends, ofcourse, on the mesh size of a given screen and may be varied as desired.The raw materials that do not pass through the screens may be recycledback to the size reduction apparatus and further reduced to properparticle size.

After separation (e.g., through screening or another separationapparatus), raw material being introduced for processing may consist ofhighly variable shapes and have a wide variety of surfacecharacteristics. These varied raw materials may include pieces that haveopen cavities, which may be susceptible to oxidation. The tendency ofmagnesium or other raw material to oxidize at high temperatures can beminimized by maintaining the raw materials in an inert atmosphere.Non-exhaustive examples of gases that can be used to create the intertatmoshphere include the noble gases such as helium, argon and nitrogen,or other gases including SF₆. The inert atmosphere may be provided byintroducing a constant flow of inert gas, such as argon, on the rawmaterial as it is introduced to the extruder. To minimize the trappingof air in the cavities, it may be desirable to tumble the raw materialsand particles over a distance in an inert atmosphere. The tumblingaction may allow the trapped air to be released and provides morecomprehensive protection against high temperature oxidation. In oneembodiment, a plurality of rotating damper-like obstructions near thefeed throat area may be used to provide the tumbling action. As thedampers rotate, the added material is forced to tumble and release anyair bubbles contained therein. Another embodiment may require no movingparts, but rather may include a sequence of projections from the feedthroat wall that also tumbles the raw material or particles that causethe release of air bubbles from the material.

Microstructural and metallurgical refinement of incoming raw materialmay be performed, at least partially, in an extruder. Extruderscontemplated by the present invention may include, for example,twin-screw extruders, single screw extruders, and stationary screwextruders. Twin-screw extruders may be either co-rotating orcounter-rotating extruders with either intermeshing or non-intermeshingscrews. In terms of design, twin-screw extruder screws may be eitheridentical or complementary. According to one preferred embodiment, theextruder may be a co-rotating intermeshing twin-screw extruder withidentical screws. In the exemplary twin and single screw extruders, ascrew may be mounted in a given barrel such that the screw is capable ofrotary movement relative to the barrel and is suitable for moving rawmaterial along the barrel and eventually forces the extrudate throughdischarge orifice. Stationary screw extruders may also be used whichincludes a screw that does not rotate relative to a barrel. In thestationary screw embodiment, a plunger arranged behind the screwrelative to the discharge orifice of the extruder may be employed toforce material along the screw and, ultimately through the nozzle of theextruder.

Screws and other components used in suitable extruders may optionallycomprise a hardening and/or a non-corrosive coating on their outersurface. Due to the high corrosion potential of processing magnesiumalloys in their semi-solid state, it is beneficial if extrudercomponents that contact semi-solid magnesium are resistant to corrosion.According to one preferred embodiment of the invention, extrudercomponents may be manufactured from a cobalt based material such as, forexample, STELLITE, a cobalt-chromium alloy, or lined with a corrosionresistant material such as, for example, SIALON, a silicon nitridealloy. Further examples of suitable corrosion resistant materialsinclude INCONEL, a nickel-chromium alloy, and HASTELLOY, anickel-molybdenum alloy, although other corrosion resistant materialsapparent to one skilled in the art may be used.

Extruder screws may be designed in a modular fashion such that severalelements line up on a shaft to produce screws of desired length. In apreferred embodiment, such elements may include, for example, high pitchforwarding elements, moderate pitch forwarding elements, low pitchforwarding elements, polygonal elements, distributive combers, kneadingblocks, and blister rings. In a preferred embodiment an aspect ratio ofa screw may be between about 20:1 and 50:1.

Extruder barrels may also be designed in a modular fashion whereinseveral segments fit together to make up the full extruder length.Barrel segments are typically manufactured in aspect ratios of 4:1,although other aspect ratios are also suitable depending on the borediameter. Barrel segments may, optionally, be provided with independentthermal control thereby providing for a plurality of longitudinallyspaced heating zones. As used herein, the term “thermal control”includes control of cooling and/or heating capabilities. Coolingcapabilities within the barrel may be provided by machined water or oilchannels. Heating capabilities may include ceramic band heaters orinduction heaters. Ceramic band heaters contemplated by the presentinvention are typically less expensive and easier to procure thaninduction heaters. Induction heaters, however, provide the advantage ofensuring that incoming alloy is heated to the core of the extruder.According to one embodiment of the invention, a given barrel maycomprise a plurality of heaters. Barrels may be manufactured such thatthe bulk metallurgy of a given barrel is one that would not react withthe raw materials, for example semi-solid magnesium. This may beaccomplished by providing a barrel lining. Non-exhaustive examples ofbarrel linings that may resist corrosion include cobalt based metal orceramic linings. Should a barrel lining be used, the lining and thebarrel desirably are closely fitted to provide more efficient heattransfer to the barrel.

In twin screw extruders, barrel segments are normally bored with twointermeshing channels, resulting in a “figure 8” pattern. Typically, thebarrel segments are bored through, with no other ports for materials toexit. In some cases, however, the barrel segment may contain an openingfor a vacuum port in order to assist in pulling off volatile componentsduring extrusion. Such an arrangement is typically called a “flashzone”.

Other than flash zone areas which may be under vacuum, all stages of theextruder for the incoming raw materials and additional alloying elementsas well as the discharge orifice may, optionally and preferably, be inan inert atmosphere. The inert atmosphere may be created by the additionof an inert gas such as, for example, argon, to prevent oxidation of thesemi-solid alloy. In order to avoid oxidation, the alloy preferably isnot exposed to ambient atmosphere until after the alloy cools to atemperature substantially below its solidus temperature. Preferably, theapparatus for producing the feedstock may be provided with means formaintaining the raw material under a cover gas in a feeder and as theraw material is fed into the extruder as particles. In addition, aninert atmosphere may be provided with means to maintain the extrudateunder an inert gas atmosphere as the extrudate exits the dischargeorifice of the extruder. For example, a hood arrangement may be used or,alternatively, a tubular ring comprising holes around its circumferencemay be used to direct streams of an inert gas onto the extrudate. Aninert gas atmosphere may serve to protect the extrudate from oxidationand/or may cool the extrudate. Preferably, the means for maintaining aninert gas atmosphere does not impede the flow of extrudate from thedischarge orifice of the extruder.

In a preferred embodiment, raw material may be introduced into theextruder at a rate less than or equal to 100% of the capacity of theextruder. The screw segments may be arranged along a shaft in a mannerwhich promotes rapid acceptance of incoming raw material into theextruder near a feed throat and then compression of the raw material asit begins to heat and become more pliable. The incoming raw material maythen be exposed to a series of kneading blocks arranged in a forwardingfashion so that dendrites present in the incoming raw material can besheared and broken. An example of a dendritic microstructure is shown inFIG. 3. Preferably, the temperature of the extruder increasescontinually along this tortuous path to a point not above the liquidustemperature of the raw material. As the alloy continues to travelforward through the extruder and to be mixed via moderate forwardingelements in the channel, it continues to increase in temperature due tofrictional heating and/or heat from a ceramic band or induction heater.With sufficient shear and heating, the raw material eventually istransformed to a semi-solid extrudate, resulting in solids having aglobular microstructure similar to that shown in FIG. 4.

The raw material may then be exposed to polygonal elements or highpitched forwarding elements within the extruder in a vacuum ventedbarrel segment, e.g. a “flash zone,” where paints, oils, and otherorganic components become volatile and may be removed by a vacuum. Thisflash zone may be used alone or in conjunction with other flashing ofthe raw material, for example, before or after reduction of the rawmaterial. A temperature profile along the length of a given extruder maybe controlled such that the temperature increase from a feeder to aflash point, decrease after the flash point and then increases again inthe direction of the discharge orifice as more clearly shown in FIG. 2.According to one preferred embodiment, the temperature profile along thelength of a given barrel is maintained such that the temperature at aflash zone represents the maximum temperature in the profile. Accordingto another embodiment, the temperature profile at the discharge orificeof the extruder represents the maximum temperature in the profile.

Additional alloying elements can be added via an additional downstreamfeed throat. Suitable alloying materials will be apparent to one skilledin the art and may include, without limitation, manganese, zinc,silicon, aluminum, chromium, cobalt, lead, platinum, titanium or acombination thereof. The raw material and any additional alloyingelements may be compressed and may pass through a series kneading blockswhere the alloying elements are dispersed into the raw material. Theresulting, newly formed alloy, then may pass through a series ofdistributive combers where it is mixed more completely under sufficientshear and temperature to maintain a semi-solid extrudate. After thismixing, the alloy may be forwarded by high pitched forwarding elements,compressed by moderate pitched forwarding elements and fully compactedby low pitched forwarding elements which forward it to the dischargeorifice. It is beneficial for the alloy to be compressed immediatelybefore the discharge nozzle so that the extrudate is a continuous strandand/or so that oxygen cannot enter and oxidize the alloy. Preferably, arate of movement of the material through the extruder is substantiallyindependent of a shearing rate of the material.

During extrusion particles of the raw material may, optionally, beintroduced into a given extruder at a rate less than the maximum ratefor that extruder. Suitable shear rates include rates of about 5 to 5000reciprocal seconds, with a rate of about 500 to about 2000 being morepreferable, and 1000 to 1500 reciprocal seconds being most preferred.Preferably, a rate of movement of raw material along a barrel of a givenextruder is substantially independent of the shearing rate of the rawmaterial. Mean residence times of material in the extruder range from,for example, about 15 to about 200 seconds. In embodiments of theinvention comprising two intermeshing screws, an exemplary, non-limitingpreferable rate of rotation for the screws is about 25 to 500 rpm, andmore preferably about 50 to about 250 rpm, and even more preferablyabout 125 to about 175 rpm.

Additional alloying elements may be added in order to resolve problemssuch as, for example, high iron content. In this particular example,manganese may be added as an alloying element because it can isolateiron in the microstructure and renew the corrosion resistance ofmagnesium if it has been lost due to ferrous contamination. Otheralloying elements contemplated by the present invention includemanganese, zinc, silicon, aluminum, chromium, cobalt, lead, platinum,and titanium. Desirably, an additional alloying element may be added tothe process while the element is at ambient temperature or at atemperature greater than an ambient temperature but less than a solidustemperature of a material in the extruder.

At the discharge orifice the material must be kept sufficiently fluid toflow out of the orifice because there is no longer the capability toshear the alloy. One way to keep the material sufficiently fluid isthrough temperature control. For example, this may be accomplished bymaintaining the temperature of the alloy in the discharge orifice at atemperature within plus or minus 5 degrees Kelvin of the intendeddischarge orifice temperature. The alloy may then be cooled quickly inthe presence of an inert gas such as, for example, argon. Optionally, agear pump constructed of corrosion resistant components may be fitted tothe discharge orifice to assist in pulling the solidifying magnesiumfrom the extruder. A die including as many as ten ports or more throughwhich material can exit may optionally be fitted to the dischargeorifice.

In the preferred embodiment, the extrudate exits the discharge orificeand/or die(s) generally in the form of a noodle-like strand or aplurality of noodle-like strands. A noodle-like strand typicallycomprises a circular cross section, which may have a diameter suitablefor final application of the feedstock (e.g., about ¼″ or less for thepreferred thixomolding operation). Depending on the final application inwhich the feedstock material may be employed, other cross sections mayalso be suitable. An apparatus according to the invention may optionallycomprise means for controlling the temperature of the extrudate. Thetemperature may be controlled by, for example heating bands in theorifice, cooling fluid in the orifice, cooling gas emanating from a ringsurrounding the orifice and/or the speed at which extrudate is pulledfrom or pushed out of the orifice. For example, the extrudate, while innoodle form, may be immediately exposed to cool argon gas to begincooling and lock a preferred microstructure into place.

Preferred microstructures may comprise, for example, a rosettestructure, a globular structure and/or an encapsulated alpha phase whichmay be preferred in a given processing application because it flows moreeasily as compared to a dendritic structure if employed in the sameprocessing application. A microstructure other than dendriticmicrostructure may also be preferred because of its physical properties,such as, for example greater tension strength and/or higher elongationas compared to dendritic materials. When the raw materials are exposedto high shear conditions during extrusion and/or just prior to dischargeinto extrudate from the nozzle, die, or other discharge orifice, thedendritic structures are generally reduced and/or eliminated. The highshear before discharge can also preferably eliminate the formation ofdendrites upon cooling the extrudate. Assuming, for the sake ofcalculation, that materials primarily exhibit laminar flowcharacteristics and Newtonian flow behavior, suitable shear rates justprior to entering the discharge orifice include, for example, shearrates of about 500 to about 2000 reciprocal seconds. Finitemicrostructures can be achieved by controlling the take-off and coolingrate from the extruder. The take off and/or cooling rates may becontrolled by the speed of extruder screws or a plunger or by take-offvia a gear pump. Take-off of the noodle may occur at speeds equal to orgreater than the output of the extruder.

At the take-off point, the extrudate may be divided (most often bycutting) into a desired final size by a pelletizer. Under differentembodiments, the extrudate exiting the extruder may be divided shortlyafter exiting or be transported by a conveyor under an inert gasatmosphere before being divided by the pelletizer. In one preferredembodiment, the pelletizer may be a mechanical cutter where the cuttingaction may be performed by a knife or blade as shown in FIG. 5.According to another embodiment, cutting may be performed by a rotarycutter as shown in FIG. 6. In order to minimize the chance ofcontamination during mechanical cutting of the extrudate, the cuttingsurface(s) may be made from the same material (e.g., same alloy) as theextrudate. A pelletizer may also be used to divide the extrudate intopellets via a non-mechanical cutter such as, for example, a laser or gasstream as shown in FIG. 7. In one embodiment, a high pressure argonstream can be used to cut the extrudate into pellets. Pellets, accordingto the invention, are preferably of uniform size, having an aspect ratioof between 0.5:1 and 4.0:1 and may, optionally, be cylindrical in shape.It should be noted that although rotary knives are suitable, they maypotentially be a source of metal contaminants in the final product asthe knives wear. Therefore, the use of like metals, a laser, or an inertgas is preferred for cutting.

Once the pellets are cooled to solidus state, they can be used asfeedstock material for molding or other forming applications. Thefeedstock material may be made from most any combination of alloys asneeded by a desired application. As discussed, the feedstock desirablyincludes a microstructure other than the dendritic microstructure whichcan be controlled by the shear and temperature during the semi-solidextrusion process. For example, magnesium alloy feedstock was preparedwith the above identified process using a stationary screw extruderhaving sufficient shear to provide a globular microstructure as shown inFIGS. 8 and 9. The extruder maintained the raw material below theliquidus temperature but greater than the solidus temperature in asemi-solid state. In another example, zinc alloy feedstock having a muchsmaller semi-solid temperature range that magnesium alloys was alsoprepared with the above-identified process under shearing andtemperature conditions suitable for formation of a globularmicrostructure as shown in FIG. 10. Control over the temperature andforward plunger speed of the extruder was closely monitored. In thefront barrel regions, a temperature tolerance range of only about 4degrees Kelvin was allowed. The discharge orifice allowed forapproximately a 10 degree Kelvin temperature range. The forward plungerspeed was held between about 0.060″ per second and about 0.120″ persecond to impart sufficient shear. The aforementioned examples arepresented to illustrate the present invention and to assist one ofordinary skill in making and using the same. The examples are notintended in any way to otherwise limit the scope of the invention.

Turning now to the drawings, FIG. 1 depicts a flow diagram for oneprocess for producing pelletized feedstock according to the presentinvention which has been described above.

FIG. 2 depicts a configuration for an extruder according to oneembodiment of the present invention. The extruder has an aspect ratio of40:1 and comprises a plurality of segments. A first feeding segmentincludes a feeding throat that is maintained at a temperature of about533° K. As the incoming raw material moves along the extruder, itsubsequently passes into a melting segment which is maintained at about811° K, a shearing segment which is maintained at about 869° K, adevolatilizing vacuum flash segment which is maintained at about 872° K,an alloying segment where alloying elements may be added which ismaintained at about 869° K, a disbursement segment which is maintainedat about 869° K, a distribution segment which is maintained at about869° K, a conveying segment which is maintained at about 869° K, apumping segment which is maintained at about 869° K and a dischargesegment which is maintained at about 872° K from which thixotropicextrudate may exit the extruder.

FIG. 3 shows a micrograph of a dendritic microstructure formed bysolidification of an extrudate not sufficiently exposed to sufficientshear forces.

FIG. 4 shows a micrograph of globular microstructure resulting fromsolidification of an extrudate exposed to sufficient shear forces.

FIG. 5 depicts an exemplary rotary knife cutter according to one aspectof the invention. In such a cutter, an extrudate strand or a pluralityof extrudate strands intersect the blades from an axial direction. Arotary knife pelletizer would be best positioned at the dischargeorifice or die of an extruder, although other positions, such as at theend of a conveyor, are also contemplated by the present invention.

FIG. 6 depicts a rotary cutter pelletizer according to anotherembodiment of the present invention. When using a rotary cutter, anextrudate strand or a plurality of extrudate strands come into contactwith the blades from a direction roughly perpendicular to an axis ofrotation of the cutters. Rotary cutter extruders may preferably be usedat a point downstream from the extruder, such as at the end of aconveyor, although rotary cutters may also be positioned at thedischarge orifice of an extruder.

FIG. 7 depicts non-mechanical pelletizers that may be used to divide theextrudate of the present invention. The non-mechanical pelletizer may bein the form of a laser or fluid stream (e.g., gas or liquid stream).

FIGS. 8, 9 and 10 are micrographs of various alloys that have beensolidified after being exposed to sufficient shear and temperature toprovide a globular microstructure.

The foregoing embodiments have been shown for illustrative purposes onlyand are not intended to limit the scope of the invention which isdefined by the claims.

1. A process for producing feedstock materials for semi-solid forming,comprising: (a) providing solid-state raw material; (b) reducing a sizeof said raw material to produce particles of raw material of a sizesuitable for further processing; (c) introducing said particles of rawmaterial into an extruder; and (d) extruding said particles of rawmaterial to form an extrudate feedstock material, wherein the extrudateis in semi-solid state.
 2. A process for producing feedstock materialsaccording to claim 1, further comprising removing magnetic contaminantsfrom said particles of raw material.
 3. A process for producingfeedstock materials according to claim 1, wherein said raw materialcomprises a magnesium alloy.
 4. A process for producing feedstockmaterials according to claim 1, wherein said raw material includes asprue, runner, ingot, scrap or a combination thereof.
 5. A process forproducing feedstock materials according to claim 1, wherein saidreducing comprises mechanical reduction of said raw material with areduction machine.
 6. A process for producing feedstock materialsaccording to claim 1, wherein reducing includes reducing said rawmaterial to particles of a first size with a first shearing machine toprovide first reduced particles, and reducing the first reducedparticles to a second size less than the first size with a secondshearing machine.
 7. A process for producing feedstock materialsaccording to claim 1, further comprising separating the particles by aphysical characteristic.
 8. A process for producing feedstock materialsaccording to claim 1, further comprising, during said extruding, addingelemental metal or an alloy to said raw material to affect themetallurgical properties of at least one of said raw material and saidextrudate.
 9. A process for producing feedstock materials according toclaim 8, wherein said metal or alloy includes manganese, zinc, silicon,aluminum, chromium, cobalt, lead, platinum, titanium or a combinationthereof.
 10. A process for producing feedstock materials according toclaim 8, wherein said metal or alloy is added at a temperature greaterthan ambient temperature but below a liquidus temperature of said rawmaterial.
 11. A process for producing feedstock materials according toclaim 1, wherein said introducing comprises introducing the particlesunder an inert atmosphere and tumbling the particles over a distance inthe inert atmosphere.
 12. A process for producing feedstock materialsaccording to claim 1, wherein said particles are introduced into saidextruder under an inert atmosphere at a rate less than a maximum ratepossible for said extruder.
 13. A process for producing feedstockmaterials according to claim 1, wherein the raw material is extruded ata rate of movement along a barrel of said extruder that is substantiallyindependent of a shearing rate of said raw material.
 14. A process forproducing feedstock materials according to claim 1, wherein saidextruding includes maintaining a shear rate of said raw material ofabout 500 to 2000 reciprocal seconds.
 15. A process for producingfeedstock materials according to claim 1, wherein said extrudingcomprises extruding at a rate of rotation of about 50 to about 250 rpmwith an extruder including two rotatable screws arranged in anintermeshing configuration.
 16. A process for producing feedstockmaterials according to claim 1, wherein said extruding comprisesextruding with an extruder comprising a flash zone between a feedingthroat and discharge orifice of said extruder.
 17. A process forproducing feedstock materials according to claim 16, wherein saidextruder includes a vacuum to draw off volatiles at said flash zone. 18.A process for producing feedstock materials according to claim 1,further comprises flashing said raw material to remove volatilecontaminants.
 19. A process for producing feedstock materials accordingto claim 1, wherein the temperature of the raw materials does not exceeda liquidus temperature during the extruding of the raw materials.
 20. Aprocess for producing feedstock materials according to claim 1, furthercomprising cooling said extrudate under an inert atmosphere.
 21. Aprocess for producing feedstock materials according to claim 1, furthercomprising pelletizing said extrudate.
 22. A process for producingfeedstock materials according to claim 21, wherein said pelletizingcomprises dividing said extrudate with a mechanical cutter.
 23. Aprocess for producing feedstock material according to claim 21, whereinsaid pelletizing comprises dividing said extrudate with a non-mechanicalcutter.
 24. A process for producing feedstock material according toclaim 1, wherein the extruder, excluding a vacuum stage, if present, isgenerally maintained under an inert atmosphere.
 25. A process forproducing feedstock material according to claim 1, wherein saidextruding further comprises exposing incoming raw material to a seriesof kneading blocks arranged in a forwarding fashion so as to providesufficient shear to remove at least a portion of the dendriticstructures present in said incoming raw material.
 26. A process forproducing feedstock material according to claim 1, wherein saidextruding further comprises exposing said raw material to polygonalelements or high pitched forwarding elements in a vacuum vented barrelsegment in order to remove volatile organic components present with saidraw material.
 27. A process for producing feedstock material accordingto claim 8, wherein said extruding, after adding the metal or alloy tothe raw materials, further comprises: (i) forwarding said raw materialwith high pitched forwarding elements; (ii) compressing said rawmaterial with moderate pitched forwarding elements; and (iii) compactingsaid raw material with low pitched forwarding elements to forward saidraw material to a discharge orifice to form the extrudate.
 28. Anextrudate produced according to the process of claim 1 having anon-dendritic microstructure.
 29. The extrudate of claim 28 wherein theextrudate includes a globular microstructure.
 30. The extrudate of claim28, wherein the extrudate includes a crystalline microstructure.
 31. Theextrudate of claim 28, wherein the extrudate is in pellet form and hasan aspect ratio between about 0.5:1 and about 4.0:1.
 32. A processaccording to claim 1, further comprising introducing said feedstockmaterial into an injection molding process.
 33. A method of injectionmolding using extrudate from the process of claim
 1. 34. An apparatusfor producing feedstock materials for semi-solid metal forming,comprising: (a) an extruder; (b) a feeder attached to and incommunication with said extruder for introducing raw material into saidextruder; (c) means for maintaining said raw material under an inertatmosphere in said feeder and as said raw material is fed into saidextruder; (d) a discharge orifice affixed to said extruder and throughwhich said raw material exits said extruder as extrudate having atemperature below the liquidus temperature of the raw material andgreater than the solidus temperature; (e) means for controlling atemperature of the raw materials within the extruder and said extrudateat less than liquidus temperature; (f) means for maintaining saidextrudate under an inert atmosphere as said extrudate exits saiddischarge orifice; and (g) a pelletizer for dividing extrudate.
 35. Anapparatus according to claim 34, wherein said extruder comprises abarrel having a plurality of longitudinally spaced heating zones and aflash zone.
 36. An apparatus according to claim 34, wherein saidextruder comprises at least one heater.
 37. An apparatus according toclaim 36, wherein said heater includes a ceramic band heater or aninduction heater.
 38. An apparatus according to claim 34, wherein saidextruder comprises a single, stationary screw mounted in said barrel.39. An apparatus according to claim 38, wherein said extruder furthercomprises a plunger for forcing raw material through said extruder alongsaid stationary screw.
 40. An apparatus according to claim 34, whereinsaid extruder comprises at least one screw that is capable of rotarymovement relative to said barrel and is suitable for moving said rawmaterial along said barrel from in the direction of the feeder to thedischarge orifice.
 41. An apparatus according to claim 34, wherein theextruder includes a barrel that includes a non-corrosive lining.
 42. Anapparatus according to claim 34, wherein the extruder includes at leastone screw having a hardening material on an outer surface of said screw.43. An apparatus according to claim 34, further comprising a gear pumpfitted to the discharge orifice nozzle of said extruder for aiding inthe removal of extrudate from said extruder.
 44. An apparatus accordingto claim 34, further comprising means for carrying extrudate exiting thedischarge orifice of said extruder away from said extruder, optionallyunder an inert atmosphere.
 45. An apparatus according to claim 34,further comprising means for maintaining all stages of said extruder,other than a vacuum stage, if present, under an inert gas atmosphere.46. An apparatus according to claim 34, wherein said pelletizercomprises a mechanical cutter with blades constructed from a compatiblealloy.
 47. An apparatus according to claim 34, wherein said pelletizercomprises a non-mechanical cutter.
 48. An apparatus according to claim34, wherein said extruder comprises a screw having a modular design suchthat a plurality of elements may be arranged along a shaft to produce ascrew of a predetermined length.
 49. An apparatus according to claim 48,wherein said elements comprise at least one of a high pitch forwardingelement, a moderate pitch forwarding element, a low pitch forwardingelement, a polygonal element, a distributive comber, a kneading block,or a blister ring.
 50. An apparatus according to claim 34, furthercomprising a size reduction apparatus for reducing a size of said rawmaterial.
 51. A system for producing feedstock materials for semi-solidforming, comprising: (a) raw material; (b) a size reduction station forreducing said raw materials into particles; (c) a separation apparatusfor removing particles by a physical characteristic; (d) an extrudercomprising a screw and containing semi-solid raw material; and (e) apelletizer.
 52. A system for producing feedstock materials according toclaim 51, wherein said separation apparatus includes a means forremoving magnetic elements.
 53. An extruder for semi-solid metalextrusion, wherein said extruder converts raw material to a feedstockand comprises a downstream feeder for introducing alloying elements tothe raw material after the raw materials have been at least partiallyprocessed in the extruder.